The present invention relates to a print controller and, particularly, to a dot-matrix print controller used in a thermal dot-matrix printer using thermo-sensitive paper or a thermal transfer dot-matrix printer.
With the recent rapid advancement in computer technology, various types of printers have been developed, one of which is the thermal dot-matrix printer. Having features of portability, simple structure, low price, low noise, and high quality printing, this printer is most often used as a keyboard printer with print data memory and an electronic typewriter.
The conventional thermal transfer printer with a buffered key-in capability will first be described broadly in connection with FIG. 1. The arrangement shown in block form in FIG. 1 includes a control circuit 1 which controls the overall printer, a keyboard 2 for entering commands and data, a document data memory 3, a display unit 4 using, for example, liquid crystal display (LCD) devices, a drive unit 5 for moving the carriage and platen, a thermal head driver 6, a thermal print head 7, a print pulse generator 8, a carriage feed motor 9, a paper feed motor 10, a head retracting plunger 11, a character generator (CG) ROM 12 storing font data, and a voltage stabilizing capacitor 13 connected between the thermal print head 7 and the power source.
The conventional thermal transfer printer with a buffered key-in capability arranged as mentioned above has some deficiencies in its print quality. The following describes the operation in which in response to the key entry of data "A", font data configured in a m-by-n dot matrix is printed in to milliseconds for each dot. In FIG. 1, the control circuit 1 responds to the key entry on the keyboard 2 to display the entered data on the display unit 4 and also stores it in the memory 3. When the printer is operated in direct print mode, the control circuit 1 makes access to the character generator (CG) ROM 12 addressed in accordance with the input data, prints the leftmost column of the m-by-n dot matrix by operating the head driver 6 in a duration of to milliseconds, and then operates on the carriage feed motor 9 to feed the carriage for one pitch rightward. These operations are repeated so that a complete m-by-n font is printed.
The aforementioned conventional print system, however, lacks in the uniformity of thickness of the print. The reason of this problem will be explained in the following.
The thermal transfer printer with the buffered key-in capability operates in different print speed in direct print mode in which a keyed character is printed immediately and in memory print mode in which keyed characters are once stored in the memory and then printed. In direct print mode, an unskilled typist will type as slow as one character per second, i.e., at a print speed of 1 cps, while a skilled typist will type as many as 10 characters per second, i.e., at a print speed of 10 cps. On the other hand, in memory print mode, in which data stored in the memory 3 are printed, characters are printed at a speed as high as 10-50 cps. On this account, during a continuous high-speed printing, successive drive pulses of a constant pulse width to causes a cumulative heating of the print head as shown in FIG. 2, resulting in an increased thickness of the print, and eventually in a transfer of image of inactive printing elements, so called "tracing". In addition to such degraded print quality, the print head can be deteriorated due to overheating. In FIG. 2, hatched portions indicate the time length for printing one character.
Furthermore, in controlling the drive of printing elements having a resistance of R, energy (v.sup.2 /R t) supplied to each element needs to be kept constant. For this purpose, it is a general convention to employ a large stabilizing capacitor 13 or a stabilized power supply in order to cope with the variable number of active elements, resulting in a bulkiness of the printer.
Some of the conventional dot-matrix printer of the serial print type have a capability of printing characters with underline. In these cases, the character printing elements and underline printing elements are arranged substantially equidistantly, and connected to the character printing elements and print signal terminals. Other ends of the character printing elements and underline printing elements are connected to the common line. On this account, in printing character "A" and an underline simultaneously, they can be printed clearly separated, whereas in printing characters "y" and "p" (lower case) with underline, the characters link with the underline, resulting in a degraded print. This linkage problem also occurs in printing lower-case characters "j", "q" and "g" with underline. This problem may be overcome by providing the conventional print head with additional printing elements so that these characters are spaced out from the underline. However, such an arrangement wastes many printing elements, and causes an imbalanced font of characters.